The present invention relates to a process for continuous casting in a continuous casting plant, in particular a steel continuous casting plant, having a continuous casting mold and a strand guide which is arranged below the discharge opening of the continuous casting mold and has rollers which support the strand on opposite sides, at least the rollers which are associated with one side of the strand being mounted on support segments which are displaceable with respect to the opposite rollers, the strand being reduced in thickness after its emergence from the mold in the manner that at least one support segment is so aligned as to form a predetermined wedge-shaped roller gap between the rollers which lie opposite each other.
In order to improve the quality of the strand, it is known to reduce the thickness of the strand directly upon its emergence from the continuous casting mold--and therefore while its core is still liquid. In order to carry out this thickness-reducing process in which the strand is reduced, for instance, from a thickness of 70 mm to a thickness of about 60 mm, it is known (EP-A 0 450 391 or DE-A 1 583 620) to establish a wedge-shaped roller gap on the strand guide. The strand, the shell of which is very thin directly below the continuous casting mold, is thereby imparted a reduction in thickness, known in the literature as a "soft reduction". In order to avoid a breaking of the strand, the zone over which a wedge-shaped roller gap is provided extends over a long length so that the strand is actually reduced in thickness as softly as possible.
From EP-A 0 545 104, a process for the continuous casting of slabs is known in which the strand is subjected to a soft reduction. In that known process, the strand travels into the path where the soft reduction is effected while it still has a liquid phase. At the end of the soft-reduction path, the strand is completely solidified. In this way, an improvement in the internal quality is to be obtained in the region of the remaining solidification, and segregations are avoided in the strand. In this case, there is the disadvantage that if the tip of the liquid center of the strand is present within the soft-reduction path, excessive forces occur between the rollers which shape the strand and the strand itself, which can lead to damage to the soft-reduction path, and particularly the rollers thereof.
The requirement that the wedge-shaped roller gap should have the greatest possible length for a soft shaping of the strand is opposed by the requirement that the strand be shaped only when it has a liquid center insofar as, particularly in the case of thin cast strands, the liquid center extends only over a relatively short length and a reduction in the casting speed can have the result that the tip of the liquid core comes into a position within the zone in which a wedge-shaped roller gap is set. In this way, excessive rolling forces can result by which the rollers which form the wedge-shaped roller gap or their roller bearings can be damaged. Another disadvantage which results from a long length of the wedge-shaped roller gap is that, upon casting, a relatively long so-called head piece of the strand is produced the thickness of which does not correspond entirely to the desired final thickness of the strand. One is forced, upon the start of the continuous casting plant, to use a dummy strand the thickness of which is adapted to the dimensions of the continuous casting mold, which, in turn, means that the strand guide must initially be set to the thickness of the dummy strand and that this thickness can only gradually be reduced by the formation of a wedge-shaped roller gap to the desired final thickness. From this, there results a solidified head piece of wedge shape, which can only constitute scrap.